There isn't any difference between American and British English from a language standpoint. Both are English. An American in the UK should be able to understand any one on the street speaking proper English and vice versa. The true difference comes in spelling, accents and vernacular. Slang develops differently in different regions, and the two regions pronounce things differently. Either way, English is English no matter if you're in the UK or the US.

American English is much easier to understand.

For me, American English is easier to understand. To be honest, i has been learning English for a long time, but i dont understand much when talking to British.